Thiamine (vitamin B-l) is known to be destroyed in the human intestine by the enzyme thiaminase I. The thiazole portion of the vitamin is replaced by a nucleophile. Bisulfite ion catalyzes a similar substitution reaction. We propose to determine the kinetics of both these substitution reactions using thiamine and thiamine-like compounds in parallel investigations involving both bisulfite ion and thiaminase I produced by Bacillus thiaminolyticus. In a recent publication (JACS, 99,3131(1977)) we advanced a mechanism for the bisulfite ion reaction. This mechanism accounts for all the currently known data and stands out as a truly novel one in thiamine chemistry. We propose that the enzyme reacts similarly. Studies are designed to establish on a molecular level the details of the enzymatic mechanism.